Today, liquid crystal display devices are used in various applications. Each of pixels in a liquid crystal display device exhibits a luminance in accordance with the level of voltage applied to a liquid crystal layer. As an electrical representation, the pixel is represented as a liquid crystal capacitance formed by pixel electrode/liquid crystal layer/counter electrode. The level of voltage applied to the pixel (liquid crystal layer) is represented based on the potential of the counter electrode. A liquid crystal material is a dielectric material, and is deteriorated when being supplied with a DC voltage for a long period of time. In order to prevent this, the polarity (direction) of the voltage (electric field) applied to the liquid crystal layer is inverted at intervals of a certain time period (referred to as the “AC driving”). Frame inversion driving (or field inversion driving), by which the polarity of the voltage (direction of the electric field) applied to each pixel is inverted at intervals of a vertical scanning period is adopted.
However, in mass-produced liquid crystal display devices, it is difficult to accurately match the absolute values of the voltage before and after the polarity inversion of voltage. Each time the polarity is inverted, the absolute value of the voltage is slightly changed. As a result, while a still image is displayed, each time the polarity is inverted, the luminance is changed and thus the display flickers. According to a method for reducing the flicker, pixels supplied with voltages of opposite polarities are located to be adjacent to each other in a display area, so that the luminances of the pixels are spatially averaged. By use of this effect, the flicker is reduced. A representative technique of such a method is “one-dot inversion driving”, by which the polarities of the voltages applied to the adjacent pixels are made opposite to each other. The term “dot” means the pixel.
In a conventionally common liquid crystal display device, one color display includes three pixels for displaying red, green and blue, which are the three primary colors of light. The luminance of each pixel is controlled to provide color display. The color display pixel and the pixel may occasionally be referred to as the “pixel” and “sub pixel”, respectively (see, for example, Patent Document 1). When one-dot inversion driving is performed in a liquid crystal display device in which one color display pixel includes R, G and B pixels arrayed in a row direction, the polarities of the pixels in one row become R(+), G(−), B(+), R(−), G(+), B(−). Namely, when the polarities of the voltages applied to pixels adjacent to each other are made inverted to each other, the polarities of the voltages applied to pixels of a same color which are adjacent to each other are also made inverted to each other.
Recently, in order to enlarge the range of colors which can be displayed by a liquid crystal display device (referred to as the “color reproduction range”), techniques of using an increased number of primary colors for display have been proposed. For example, Patent Document 1 discloses a liquid crystal display device in which a color display pixel includes a red (R) pixel, a green (G) pixel, a blue (B) pixel, and a pixel of at least one more color (yellow (Y) pixel, a cyan (C) pixel, a magenta (M) pixel or a white (W) pixel). When the white pixel is used, the color reproduction range cannot be enlarged but the display luminance can be increased.
As shown in FIG. 8(a) of Patent Document 1, when one-dot inversion driving is performed in a liquid crystal display device in which two color pixels are alternately arrayed in the row direction, the polarities of the pixels in one row become, for example, R(+), Y(−), R(+), Y(−). Namely, pixels of the same color are supplied with voltages of the same polarity. When display is provided with pixels of a particular color (e.g., when red is displayed in the entire screen), flicker occurs.
Patent Document 1 describes that flicker can be prevented by the following. A color display pixel includes four or more pixels, which include N pixels (N is an integer of 2 or greater) at least in a vertical scanning direction (column direction). Two-dot inversion driving (by which the polarity is inverted every second column of pixels) is performed in a horizontal scanning direction (row direction), and N horizontal line inversion driving (by which the polarity is inverted every N'th row of pixels) is performed in the vertical scanning direction.
Patent Document 2 also discloses a liquid crystal display device in which a color display pixel includes pixels arrayed in two rows by row columns, although this is not for the purpose of enlarging the color reproduction range (in Patent Document 2, the color display pixel is referred to as the “picture element”). As examples of combination of four pixels included in the color display pixel, Patent Document 2 shows a combination of one blue pixel, one red pixel and two green pixels, and a combination of one blue pixel, one red pixel, one green pixel and one white pixel. The liquid crystal display device disclosed in Patent Document 2 includes one scanning line common to the four pixels included in the color display pixel and four signal lines (two signal lines are provided in each of two positions sandwiching two pixels arrayed in the column direction). The polarities of the voltages supplied to adjacent signal lines are opposite to each other. Signal lines connected to pixels located at corresponding positions in color display pixels adjacent to each other in the row direction have different positional relationships from each other with respect to the corresponding pixels. As a result, the polarities of the signal voltages supplied to pixels of a same color which are adjacent to each other in the row direction are opposite to each other. Therefore, flicker can be prevented.